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CREVICE CORROSION PERFORMANCE OF SOME COMMERCIAL STAINLESS STEELS IN ARABIAN GULF SEAWATER1

Anees U. Malik, Nadeem A. Siddiqi and Ismaeel N. Andijani ABSTRACT Crevice corrosion tests were carried out to evaluate the perjormance of some conventional and high alloy stainless steels in Arabian Gulf seawater at 25oC and 50oC using specimens having three different surface conditions namely as received wheel ground and 180 grit SiC ground. Immersion tests of 150-180 days duration and accelerated tests were employed to investigate the crevice corrosion behavior. Immersion test results show that crevice corrosion of 3127 hMo, Remanit 4565, 654 SMO, and Monit 44635 initiated as surficial corrosion with virtually no measurable depth of attack; 254 SMO, Duplex 2205, Remanit 4575, 904L and 317L corroded on l-2 sites attack. Crevice corrosion attack was most predominant at 50o C in 180 grit SiC finished surfaces and there was no evidence of crevice corrosion attack in as received samples at room temperature. A method based on Oldfield and Sutton9 mathematical modelling of corrosion in chloride media has been applied for determining critical crevice solution pH (CCSpH), an important parameter which delineate the transition between passive and active states. The CCSpH appears to be a linear function of PREN indicating the strong injluence of Cr, MO and N additions on the crevice corrosion characteristics of steels. INTRODUCTION Stainless steels containing at least 12% Cr with some allolying elements additions in varying concentrations, can provide an extraordinary range of corrosion resistance due to the passive film existing on the metals. Passivity can be maintained depending on the environment and on the composition of the stainless steel. Seawater is quite a hostile environment for most engineering materials and chloride in seawater is the most detrimental chemical species initiating a corrosion attack. Stainless steels gen-

1 Issued as Technical Report No. SWCC (RDC)-28 in August, 1993

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erally show good resistance in seawater but chloride ions may disrupt the passive film resulting in localized corrosion. The avoidance of such localized attack is the focus of most of the studies involved in selection of an appropriate stainless steel. Crevice corrosion is one of the common forms of localized corrosion and is the most troublesome problem in marine structures, pipelines, pumps and shafts, valves and the situations where there is a possibility of water stagnation [l-4]. Crevice corrosion involves a number of simultaneous and interacting processes including mass transfer, formation of metal ions within the crevice solution and hydrolysis reactions, culminating in a critical crevice solution. Active corrosion occurs at this point and the corrosion propagates rapidly [5]. A theoretical definition of the critical crevice solution is the pH and the chloride concentration at which the hydrogen evolution current is less than the anodic peak current at the anodic peak potential. Therefore, when a metal is polarized in such a solution, an anodic peak will occur on the polarization scan. A number of empirical methods of determining the critical crevice solution pH (CCSpH) have been cited in the literature [6-91] the method proposed by Oldfield et al [9] has been widely used and it takes the pH at which active current reaches 10A cm-2. In another paper Oldfield [10] used active peak current of 5 A cm-2 for determining CCSpH. Because of the superior characteristics of stainless steels to other marine construction materials, efforts have been made to improve the resistance of stainless steels to localized corrosion. The alloying elements that increase the resistance of stainless steel to pitting and crevice corrosion are chromium, molybdenum and nitrogen, the improvement in the corrosion resistance can be quantitatively estimated by the magnitude of pitting resistance equivalent, PREN represented by the formula [ll]: PRE N = %Cr + 3.3 x %Mo + 16x%N Hack [2] studied the crevice corrosion behavior of a number of austenitic and ferritic steels by exposing to filtered seawater at 30o C for 30 days using multiple crevice washers. It was found that some of the alloys are'not only more resistant than AISI 316 but also show resistance equivalent to more costly Ni-base alloys. Austenitic alloys studied required 8% Mo to prevent crevice corrosion in these exposures while ferritic alloys required 25% Cr and about 3.5% Mo. Oldfield 10 studied the influence of N, Mn and S on the crevice corrosion resistance of a large number of commercial and high purity experimental stainless steels in marine environments using an accelerated test technique. Amongst commercial,alloys, UNS 31254 gave. the best corrosion resistance. It has been found that if Mn is present at levelsless than 0.5%

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the corrosion resistance is increased significantly and there is little effect on resistance to crevice corrosion initiation between 0.5% and 1.5%. Decreasing S level increases resistant to corrosion. Oldfield [4] studied the effect of temperature and surface roughness on the seawater crevice corrosion resistance of the commercial and experimental austenitic stainless steels including 316 and 20 Cr-6 Mo types containing two ranges of nickel contents, 10% to 30% and 20% to 40%. The results show that the resistance to crevice corrosion initiation was better in rough surfaces in comparison to ground or highly polished surfaces. Moreover, there is an improvement in resistance to crevice corrosion initiation as the temperature decreases from 70oC to 5oC, but little influence of the variation in nickel contents. For corrosion propagation, nickel has been found extremely beneficial in improving the resistance of 316 type steels but in 20% Cr-6% Mo experimental alloys, Ni above 20% did not significantly effect corrosion rates. These results indicate the feasibility of significantly improving the corrosion resistance of high alloy stainless steel by adjusting the composition so that they could be used for seawater applications. This report presents the results of an investigation carried out to study the crevice corrosion resistance behavior of conventional and high alloy stainless steels in Arabian Gulf seawater. The main aim of the investigation is to study the effect of the temperature, surface condition and dominant alloying additions e.g. Cr, Ni and Mo on the crevice corrosion characteristics of the stainless steels in marine environment using exposure and accelerated test techniques. EXPERIMENTAL Materials Conventional stainless steels (304L, 316L, 317L, 904L) and high alloy stainless steels (254 SMO, 654 SMO, 1925 hMo, 3127 hMo, Remanit 4565, Remanit 4575, Monit 44635 and Duplex 2205) were obtained commercially in sheet forms. The flat specimens were used during the experiment without any further heat treatment. The chemical compositions of the alloys are given in Table 1. Gulf seawater was collected in bulk amounts from intake point of Al-Jubail desalination plant and was stored in large polyetheylene containers. The chemical composition of the seawater and its important characteristics are given in Table 2.

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Technique & Procedure Exposure tests were carried out on conventional and high alloy stainless steels in natural seawater at 50oC and at room temperature. Three surface conditions were selected i.e. wheel ground, 180 grit SiC ground and as received surface. The alloys were cut to approximately 100 x 70 mm with a 12 mm hole being drilled through the

center.

The crevice corrosion tests assembly was consisted of stainless steel nut, bolt and washers and two "Teflon" crevice formers with 6 mm annuli. The nut and bolts were covered by Belzona Elastomer. In all tests carried out in this work the assembly was torqued to 9.49 Nm (7 ft lb). Accelerated Tests: To complement the exposure testing, accelerated testing for resistance to crevice corrosion initiation was carried out. The specimens used were in the rod or sheet form. A wire was soldered to one end for electrical connection and the whole unit was mounted in epoxy resin to provide a crevice free mount. The exposed end was grounded to 600 grit SiC finish and rinsed with distilled water. All the specimens were again polished to the finish before starting each test. All the experiments were carried out using a corrosion cell with saturated calomel electrode (SCE) with potassium chloride salt bridge as a reference and graphite as a counter electrode (EG & G Model K 0047). Deaeration of the solution was achieved by bubbling high purity nitrogen through a filter stick. The test cell was heated on a mantle with the test solution maintained at 50°C. The test solutions were made up with natural seawater with sodium chloride additions that varied according to the working pH value Table 3. The pH adjustments were made immediately before each experiment using hydrochloric acid. After polishing, the specimens were left for 30 minutes in the air and were then passivated by immersion for 15 minutes in 17% HN03 before each experiment. The value of critical corrosion solution (CCS) is determined through a series of potentiodynamic polarization experiments in which the simulated crevice solution is continuously deaerated and is of increasing aggressiveness according to Table 3. The electrodes were exposed to the electrolyte and polarized at - 600 mV. The potential was then changed at a rate of 10 mV/s until a potential of + 100 mV (SCE) was obtained. The anodic peak height in current density units is recorded for a given

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chloride concentration and pH. The critical solution is taken to be that which first produces an anodic current peak of 10 µA cm-2 (10 µA cm-2 is considered to be equal to a corrosion penetration rate of -0.09 mm/year). RESULTS AND DISCUSSION Exposure Tests The results of the exposure tests carried out on the wheel ground, 180 grit SiC ground and as received samples exposed to natural seawater at 50°C and room temperature are summarized in Tables 4-9. For wheel ground samples at 50°C, out of 11 conventional and high alloyed steels, 4 did not corrode at all. The maximum depth of attack was in SS 304L. Alloy 3127 hMo, Monit 44635, Remanit 4575 and Remanit 4565 showed only surface corrosion. In all cases the breaking down of the passive film took less than 24 hours except Monit 44635 and Duplex 2205. For 180 grit SiC ground samples at 50oC, two of the high alloyed steels corroded e.g. 254 SMO and Remanit 4575, at one site attack only. 304L and 316L corroded in 9 and 8 sites attack, respectively; 904L also corrode in 2 sites attack. The rest of the alloys corroded as surficial. For as received samples at 50oC except 654 SMO, 3127 hMo and Duplex 2205, all other alloys corroded. SS 904L and 254 SMO corroded only as surficial. The maximum depth of attack was in SS 304L. Except AISI 304L and 316L steels, none of the alloys corroded as measurable depth of attack at room temperature under the three different surface conditions. 304L corroded under wheel ground and 180 grit SiC ground surface conditions. 316L corroded only under wheel ground surface condition at a high maximum depth of attack. Accelerated Tests Polarization curves have been obtained for both the conventional and highly alloyed steels as functions of the electrolyte aggressiveness. The excellent resistance of high alloyed steels was reflected by the experimental results, in which the lower concentrations of added chloride ions in natural seawater produced no appreciable corrosion current. For getting measurable anodic peak heights, the most aggressive solution was needed together with low pH values. In each case the critical solution composition is taken to be that which first produces an active peak current of 10mA cm-2, this being equivalent to a corrosion penetration rate of ~ 0.09 mm/year.

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Figure 1 shows variation of active peak height (APH) with CCS pH value for different alloys in natural seawater plus Nacl solution at 50oC. Critical value is that solution which produces APH equivalent to 10 cm-2. Values obtained for the critical crevice solution (CCS) of the alloys are given in terms of ranking in the Figure 2 starting with SS 304L and ending with 3127 hMo. Crevice geometry, specific environments and surface conditions are the important factors in determining resistance to crevice corrosion. Greater is the magnitude of the parameters like tightness, chloride solution concentration, temperature or surface finish, more severe is the crevice corrosion. Keeping in view the profound influence of the above mentioned factors on the crevice corrosion, tests were performed at a crevice assembly torque of 9.49 Nm and varying the other parameters using a 180 grit SiC finished surface and a temperature of 50oC. The exposure test results of high alloy stainless steels showed good crevice corrosion resistance. As received surface samples at 25oC solution temperature showed no crevice corrosion for high alloy as well as conventional stainless steels within a period of 180 days, which indicates that in this condition, crevices do not act as sites for corrosion initiation. However, at 50oC, except 654 SMO, 3127 hMo and Duplex 2205 all other alloys corroded. The maximum depth of attack was in SS 304L, but the crevice corrosion index CCI (CC1 = number of site attack x maximum depth of attack) was more in SS 316L as the number of sites attack in SS 316L(4) was more as compared to 2 sites attack in SS 304L. Highly alloyed steel hMo 1925 suffered corrosion on 5 sites, but the maximum depth of attack was 0.05 mm with 0.25 CCI. SS904L and 254 SMo corroded only surficially. It was noticed that at room temperature, 180 grit SiC finished specimens did not corrode to a measurable depth of attack except SS 304L, which has a very small yet measurable crevice corrosion depth. It implies that the conventional stainless steels could resist the seawater crevice corrosion under controlled conditions. Under wheel ground surface conditions at room temperature, only 304L and 316L gave considerable corrosion depths while the maximum depth of attack was in 316L but the number of sites attack in 304L (6) was more than in 316L (2) and hence the crevice corrosion index (CCI) is higher in case of 304L. This mean that the crevice corrosion propagation of 304L is high because of the absence of molybdneum content. A very small crevice corrosion product build up was observed on 317L, 904L, Remanit 4565, Monit 44635,1925 hMo, 3127 hMo, Remanit 4575 and Duplex 2205 at room temperature and on 3127 hMo, 1925 hMo, 654 SMO, Monit 44635, and Remanit 4565 only at 50oC, however after crevice formers were disassembled and samples were cleaned, no measurable depth of attack was found when corroded areas were examined microscopically. This is because of the high propagation resistance of

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the alloys due to the strong repassivation of the protective passive film on the metal surface. Therefore, the crevice corrosion initiation of an alloy does not necessarily relate to its propagation rate. From the data it appears that at 50oC, 180 grit higher crevice attacks than the similar surface At 50oC, 304L, 316L and 904L corroded on 8, Remanit 4575 and 254 SMO corroded on one 304L corroded. SiC finished specimens suffered much finished samples at room temperature. 9 and 2 sites attack, respectively, and site attack. At room temperature, only

The results of wheel ground samples at 50oC showed the corrosion of 3 alloys namely, 304L (0.12), 316L (0.09) and Duplex 2205 (0.204) whereas at room temperature the first two alloys with the same surface finish suffered corrosion, e.g.: 304L (0.24) and 316L (0.46). However, the crevice attack on 304L and 316L at room temperature appears to be more than at 50oC; this perhaps could be attributed to shorter exposure time (30 days less) at 50oC. The alloys which show only surficial corrosion in any conditions have high PREN e.g., 654 SMO (56.5), 3127 hMo (48.2), 1925 hMo (43.9), Remanit 4565 (48) etc. Qualitatively speaking, stainless steels having high Cr, Mo and N contents or PREN greater than 30 are more resistant to crevice corrosion. The plots of active peak height (APH) and CCSpH values for steels in natural seawater and NaCl solution indicate lowest CCSpH for 3127 hMo (0.481) and 654 SMO (0.50) and highest for 304L (2.298) and 316L (1.785). These results show the excellent resistance of superaustenitic 3127 hMo and 654 SMO towards more aggressive crevice forming solutions and vulnerability of 304L to crevice attack. The lower slopes of the former indicate lower corrosion rates. Plots of PREN and CCSpH of conventional and high alloy steels show a linear relationship (Fig. 3) indicating a strong but negative dependence of critical crevice solution aggressivity on Cr, Mo and N contents. A comparative study of the results of exposure and accelerated corrosion tests in seawater indicates that the stainless steels show similar crevice corrosion behavior under the two test conditions. CONCLUSION (1) The high alloy stainless steels have much better crevice corrosion resistance than conventional stainless steels.

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(2) Besides superstainless steels, conventional stainless steels containing > 6 Mo% can be used with little chances of corrosion for seawater applications involving crevice forming systems. (3) Crevice corrosion could initiate in high alloy stainless steels but usually a propagation step is not followed. (4) CCSpH appear to be a linear function of PREN showing a strong but negative dependence of critical solution aggressivity on Cr, Mo and N contents of the

steels.

(5) Both exposure and accelerated corrosion tests provide similar informations regarding the crevice corrosion behavior of steels in seawater. (6) At a constant torque, the surface finish (from rough to smoother) and increasing temperature enhance the possibility of corrosion in crevice forming system. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. R.W.Stachle, B.F.Brown, J. Kruger, A. Agrawal, " Localized Corrosion", NACE, Houston, TX. (1974). H.P.Hack, " Materials Performance", 22, 24 (1983). R.M.Kain, "Corrosion", 40,313 (1984). J.W.Oldfield, "Crevice Corrosion of Stainless Steels in Seawater," Acorn No.1, pp l-8 (1988). M. Watson and J. Postelthwaite, "Corrosion", 46,522 (1990). J.M.Defranous and R. Tricot, " Memoires Scientific Revue Metallurgique", 69, 317 (1992). J.L.Crolet, J.M.Defranous, L. Seraphin and R.Tricot, ibid, 71,797 (1974). P.Sury, " Corrosion Science", 20,291 (1980). J.W.Oldfield and W.H.Sutton, " British Corrosion Journal", l5,31 (1980). J.W.Oldfield, "Corrosion", 46,574 (1990). G.Herbslab,"Werkstoffe und Korrosion", 33,334 (1982).

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